Priority is claimed to Japanese Patent Application Number JP2006-048726 filed on Feb. 24, 2006, the disclosures of which are incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a semiconductor device intended to improve high frequency characteristics and a current amplification factor of a transistor.
2. Description of the Related Art
As an example of a conventional semiconductor device and a method of manufacturing the same, the following NPN transistor has been known. An N type epitaxial layer is formed on a P type silicon semiconductor substrate. In the epitaxial layer, an external base region is formed so as to surround an active base region. The external base region is connected to an external base lead-out electrode formed on the epitaxial layer. In addition, an emitter region is formed in the active base region, and is connected to an emitter lead-out electrode. Moreover, a collector region is formed in the epitaxial layer. In this event, an impurity in the external base lead-out electrode is diffused into the epitaxial layer to form the external base region. Moreover, the active base region is formed by implanting ions of an impurity from the surface of the epitaxial layer into a region where the external base lead-out electrode has an opening. Specifically, in the conventional NPN transistor, improvement in high frequency characteristics is achieved by forming the active base region having a small diffusion depth inside the external base region. This technology is described for instance in Japanese Patent Application Publication No. Hei 10 (1998)-303209 (pp. 4-8, FIGS. 1, 10 to 12).
As another example of the conventional semiconductor device, the following NPN transistor has been known. An N type epitaxial layer is formed on a P type silicon semiconductor substrate. A high-concentration N type buried diffusion layer is formed so as to extend in both the P type silicon semiconductor substrate and the N type epitaxial layer. Moreover, in the N type epitaxial layer, a low-concentration P type diffusion layer as an internal base region and an N type diffusion layer as a collector region are formed. In the P type diffusion layer as the internal base region, a high-concentration P type diffusion layer as an external base region and an N type diffusion layer as an emitter region are formed. This technology is described for instance in Japanese Patent Application Publication No. 2000-260891 (pp. 5-6, FIG. 1).
In the conventional semiconductor device, a base resistance value is reduced by forming the emitter region so as to overlap the high-concentration active base region having a small diffusion depth. Thereby, the high frequency characteristics and a current amplification factor of the NPN transistor are improved. However, there is a problem that formation of the high-concentration active base region having a small diffusion depth makes it difficult to obtain desired breakdown voltage characteristics (VCEO(V)) in the NPN transistor.
Moreover, in the conventional semiconductor device, the high-concentration diffusion layer as the external base region and the diffusion layer as the emitter region are formed in the low-concentration diffusion layer as the internal base region. Specifically, by forming the low-concentration diffusion layer as the internal base region, desired breakdown voltage characteristics (VCEO(V)) of the NPN transistor are achieved. However, since the diffusion layer as the internal base region has a low concentration and a large diffusion depth, a base resistance value is increased. Accordingly, there is a problem that it is difficult to obtain desired high frequency characteristics and a desired current amplification factor of the NPN transistor.